nakadashi 中出し

In Japanese, nakadashi 中出し means literally "releasing inside," but it's often used as a sexual term: "ejaculating inside," or "internal ejaculation."

Basically, it means ejaculating during intercourse without a condom, inside the partner's body. The opposite is sotodashi 外出し, "releasing outside."

Some dictionaries translate nakadashi as "intravaginal ejaculation," or "creampie," however, it's not exactly the same thing. The word nakadashi means just "inside," it isn't necessarily inside the vagina, it isn't necessarily semen that's being released inside, and it' isn't necessarily sexual at all.

But normally it's sexual, it's semen, and it's in the vagina.

Unambiguously, the term would be chitsu-nai shasei 膣内射精, "vagina-inside ejaculation."